In the manufacture of multilayer ceramic (MLC) substrates for integrated circuit semiconductor package structures, a plurality of green ceramic sheets is formed by doctor blading a slurry containing a resin binder, a particulate ceramic material, solvents, and a plasticizer, drying the doctor bladed sheet, and cutting it into appropriate size sheets. Via holes are then punched for forming electrical interconnections through the sheet. Electrically conductive paste is deposited in the holes and in wiring patterns on the surface of the sheets, the sheets stacked and laminated, and the assembly subsequently fired at a sintering temperature. Punching the via holes in ceramic sheets presents formidable engineering problems in view of their small size and density and the complex hole patterns needed. It is convenient to punch via holes with apparatus of the type disclosed in Lucas et al., IBM Technical Disclosure Bulletin Vol. 13, No. 9, February, 1971, p. 2536, Kranik et al, IBM TDB Vol. 16, No. 12, May, 1974, p. 3933-34, and Kranik et al, U.S. Pat. No. 4,425,829, the disclosures of which are incorporated by reference herein. In these apparatus a plurality of punch elements arranged in a grid on a punch head are indexed over the green sheet which is covered by an interposer mask. The interposer mask contains openings where holes are desired to be punched. When the punch elements contact the interposer mask as the punch head is moved downwardly a hole will be punched where the openings occur since the punch element will pass through the openings in the interposer mask, and through the ceramic green sheet. In other areas covered by the interposer mask, i.e. where holes are not desired, the interposer mask will cause the punch element to be retracted into the head. The sheet is sequentially indexed through a predetermined number of positions to complete the punching of a sheet.
Automated punch apparatus which utilized individually programmable punches have been disclosed in Cochran et al., IBM TDB Vol. 20, No.4, September, 1977, p. 1379, and in Stroms, U.S. Pat. No. 4,872,381, the disclosures of which are incorporated by reference herein. This type of punching apparatus does not require the above described interposer mask, since the individual punching elements can be sequentially activated upon command.
It is essential that the punching operation be rapidly and accurately performed, as well as producing products free from defects. Each green sheet can contain over 100,000 punched holes. A single defect can potentially render a green sheet unsuitable for further processing. Of particular concern is the adherence to the tip of the punch of a slug punched from the sheet. The inherent adhesion characteristics of the unfired green sheet are amplified by the large punching force applied over the small area of the punch tip. The diameter of the punch tip can be as small as 5 to 6 mils in current applications and is expected to be 4 mils or less for advanced substrates, resulting in a pressure at the punch tip on the order of 2700 kg/cm.sup.2. If the punch slug adheres to the punch it may be drawn back into the punched hole, causing a substrate defect. To eliminate the likelihood of such defects, it has been standard practice to use two punch strokes for each hole. This practice greatly increases green sheet processing time, however.
The problem of slug adhesion to the punch is not limited to the punching of ceramic green sheets and has been discussed in other punching application references. One method used in punching apparatus for the removal of punch slugs is the use of either pressurized air or a vacuum to force the slug from the punch. Goldman, U.S. Pat. No. 3,524,368, and Adams et al., U.S. Pat. No. 3,580,120, disclose apparatus in which air is channeled through the punch to remove the slug from the tip of the punch. This method is not practical for the punching of extremely small diameter holes, however.
Other applications either direct air into or apply a vacuum to a chamber below the punch to clear the slugs and do not directly address the problem of slug adherence. Examples of such applications are shown in Kirkowski, U.S. Pat. No. 3,104,804; Scott et al., U.S. Pat. No. 3,800,643; and Sickel, U.S. Pat. No. 3,602,080. Keyes et al., U.S. Pat. 3,710,666, does address the adhesion problem and discloses the injection of air into a series of inclined flow passages in a die to produce a whirlpool effect to remove slugs from the punch.
The use of air flow slug removal methods in ceramic green sheet punching to achieve single stroke punching has been attempted. Kranik et al., U.S. Pat. No. 4,425,829, discloses a tube protruding into the die bushing which upwardly injects air into the die cavity below the punching area. This air flow induces circulation in the die bushing cavity which assists in forcibly removing slugs from the punch. This arrangement does not provide the repeatability necessary to achieve single stroke punching.
Stroms, U.S. Pat. No. 4,872,381, discloses injecting air from the die housing through a passage in the die bushing to remove slugs from the punch. This arrangement has also not provided the repeatability necessary to achieve single stroke punching. FIG. 1 is a detailed view of the punch interface of the prior art Stroms apparatus. The usable length "a" of punch 10 is limited by the difficulty of fabricating an accurate, small diameter punch suitable for the rigors of production punching and by the requirement of machining radius "b." This usable length must extend through wall thickness "c" of punch bushing 11, workpiece 12 thickness "d," and the wall thickness "e" of die bushing 13. The accumulation of these dimensions and accompanying tolerances results in a minimal protrusion of the punch beyond surface 15. It is very difficult to cause passage 17 to direct air precisely at surface 15 because of the required wall thicknesses and tolerances involved. In particular, the necessary alignment of bushing passage 17 with housing passage 16 creates tolerance problems. The above described problem is aggravated in a non-programmable multiple punch which additionally requires an interposer thickness.
While both the Kranik and Stroms configurations have been useful in removing slugs from the punch tip, neither configuration provides the 100% slug removal which is required for single stroke punching. A 99.9% slug removal rate on a green sheet containing 100,000 holes results in 100 defects per sheet, any one of which renders the sheet unacceptable for further processing. The problem can be further appreciated by considering that a defect may not be detected until the green sheet is laminated into a substrate containing 60 or more layers.